Optical instruments are used for surveying purposes and are supported on, and attached to, the upper end of a tripod by means of a tribrach device. The tribrach is secured on its underside to the upper end of the tripod, and the optical sighting instrument is detachably secured to the top of the tribrach device.
With a conventional tribrach device, the three depending legs of an optical sighting device are received in three openings in the tribrach, after which the legs are locked by rotating a small tab or lever to move a cam against the legs in the openings. One of the three openings in the tribrach is elongated to compensate for any irregularities in centering the device in the tribrach. This allows the device being fastened to the tribrach to deviate in position. Such a mechanism for locking the legs in the tribrach device is cumbersome and often fails to provide a secure locking of the legs in the tribrach. Thus, during windy conditions, the optical instrument can become loosened because the lock mechanism does not securely hold the legs of the instrument in the receiving openings in the tribrach. Also, the lever on the locking mechanism is difficult to operate while wearing gloves (e.g., during cold weather).
Another problem associated with the use of optical sighting instruments is that they must be vertically aligned over a control point (e.g., a ground mark, stake, monument, etc.). This can be a time-consuming procedure.
Recently, U.S. Pat. No. 5,159,760 described a device for aligning a geodetic instrument over a defined ground mark using a collimated laser beam in a manner such that the vertical distance between the defined ground mark and the sighting line of the geodetic instrument can be measured. Such device does not cure the problems associated with locking the geodetic instrument to the tribrach. Also, the device is not suitable for use in subterranean situations where it is necessary to align an optical sighting device vertically beneath a marker which is located above the instrument (e.g., where the marker is located on the ceiling of a tunnel).
Yet another problem or disadvantage which is common in conventional tribrachs is that they are composed of a cast material which is prone to temperature fluctuations and are also easily damaged (even with minor impacts).
U.S. Pat. No. 3,471,234 describes a laser beam projector for use in surveying operations. Such patent does not describe a tribrach having the advantages provided by the present invention.
U.S. Pat. No. 4,309,010 describes a tripod with an adjustable support surface for supporting various types of objects(e.g., cameras, projection apparatus, geodetic instruments, antennas, etc.). This patent does not describe a tribrach having the advantages provided by this invention.
U.S. Pat. No. 4,832,296 describes a giant tripod assembly for supporting laser beam generators. This patent does not describe a tribrach of the type provided by this invention.
There has not heretofore been provided a tribrach device having the advantages provided by the present invention.